Heteropoda.-The symmetrically convolute Bellerophon, and its allies Cyrtolites and Porcellia, are confined to Paleozoic Strata. The recent Carinariæ are represented by one tertiary Italian fossil. Gasteropoda.-The marine kinds-all breathing by gills and freely swimming in their embryonic state by help of two ciliated fins, retractile with the body into a convolute shell-grow by constant laws of development into shells of various forms, usually spiral, though examples occur in which this character is insensible (e. g. Patella). The most numerous order both in the recent and fossil state receives the title of Prosobranchiata, from the anterior position of the gills. This character cannot be recognized in fossils, but the forms of the shells are always sufficient to identify the order. It may be divided into two suborders Holostomata with entire aperture to the shell, Siphonostomata with the aperture notched or canaliculate. The former are for the most part feeders on vegetables, the latter, for the most part carnivorous. The late Mr Dillwyn remarked the prevalence of the siphonostomatous shells in the Tertiary Strata, and the extreme rarity of them in older strata. This curious generalization is found to be of much importance in general views regarding the succession of Molluscous life. No siphonostomatous shell has yet been found R. L. H in the Paleozoic Strata. If we place the Mesozoic fossil shells, formerly called Rostellaria (now Alaria), and Cerithium, in the holostomatous division-as Morris and Woodward do-there will remain but few exceptions to the rule that the Paleozoic and Mesozoic Gasteropoda belong to the herbivorous division. Euomphalus and Murchisonia are Palæozoic; Alaria and Nerinæa, Mesozoic. The function of the Carnivora was in the earliest of these periods principally exercised by Cephalopoda; in the later period Fishes and Reptiles were effective as allies, or opponents. Cephalopoda, among the most abundant as well as highest in organization of all the fossil mollusca, are much less numerous in the modern than they were in the older periods. If we class them by the organs usually called arms or feet, as Octopod, Decapod, and Polypod, we find no trace of the first in the Strata of the British Isles, though Argonauta is fossil in the Italian Tertiaries. Decapod fossil genera more or less allied to the recent Loligo, which includes a long horny pen; to the recent Sepia, which contains a broad calcareous plate, thickly fibrous in front, concave behind, and ending in a solid apex; and to the fossil Belemnite, which is of a long conical figure, concave and chambered in front, fibrous behind, and sometimes mucronated. These are all absent from the Paleozoic Strata, and Belemnites, by far the most numerous group, are peculiar to the Oolitic and Cretaceous formations. The recent genera have an ink-bag containing black pigment; this organ is recognized in several of the fossil races, filled with the fossil ink, which retains its good colour, and has been successfully employed in drawings. The Polypod genera were (unlike all those previously mentioned except Argonauta) protected by an external shell; in these it was concamerated, the chambers formed by transverse plates which were pierced by a pipe opening to the last or outer chamber, into which the whole or principal part of the animal could be retracted. It is usual to rank them in three families-Orthoceratidæ, Nautilidæ, Ammonitidæ. Of these, the first did not pass1 the Paleozoic period; Ammonitidæ did not pass the Mesozoic period; but Nautilidæ, which began in the earliest ages, lived in all the subsequent seas, and still adorn, though not plentifully, the modern ocean. Among the Nautilidæ, however, Clymenia ceased before the close of the Palæozoic age. The geological range of the Ammonitida is still more 1 Unless in the single case of the Triassic beds of Saint Cassian, which contain what seem to be Orthoceratites, and also very peculiar Ammonitidæ having the numerous gradually diminishing lobes of Ceratites, with the highly ramified sutures of the Oolitic Ammonites. H 2 curious, for the earliest group, Goniatites, ceases with the upper Palæozoic Strata; Ceratites occurs in Muschelkalk, but does not enter the Lias; Ammonites begins at some height in the Lias, and dies out before reaching the upper Chalk, being accompanied in decline by Hamites, Scaphites, Turrilites and Baculites. These distributions appear in the following Table. Ammonites sublævis. Ceratites nodosus. ↑ m Goniatites striatus. |